2d vs 3d Depth of Field
I ran some tests with 100 samples each with this scene:


Defocus Node:


Time: 2:01 + roughly 7 seconds for compositing = 2:08
Cycles DoF:


Time: 2:02
Conclusions:
Based on the above results, rendered DoF seems more reliable and realistic, as well as a little bit faster in this case.
The main advantage of composited DoF is the ability to tweak the settings after the render.